1. Field of the Invention
The present invention relates to a solid immersion lens (SIL). The present invention also relates to a focusing lens, an optical pickup device, and a data recording/reproducing apparatus, each of which uses the solid immersion lens.
2. Description of the Related Art
Data-recording media, such as those with card and disc shapes, have been widely used as storage media for audio data, video data, other kinds of data, programs, and the like. Systems for recording data on these data-recording media and/or reproducing data therefrom may include an objective lens that faces the surface of the data-recording medium in a noncontact manner. The objective lens reads minute recording marks by detecting a reflectivity change on a phase change material or minute irregularities formed in the recording surface of the data-recording medium or by detecting a magnetic domain with a change in Kerr rotation angle in the case of a magneto-optical recording system. The data-recording media include optical recording media, magneto-optical recording media, and magnetic recording media using a light-assisted magnetic recording system in which the coercivity of a magnetic recording layer is reduced by light irradiation. In this specification, the term “data-recording media” includes such optical recording media, magneto-optical recording media, and magnetic recording media using the light-assisted magnetic recording system.
Such data-recording media have been desired to have larger capacities and higher recording densities. Therefore, a technology for forming a minute light spot corresponding to a smaller recording mark and reading the recording mark with high resolution has been studied.
The size of the light spot on a data-recording medium can be almost equal to the value of “λ/NAobj”, where “λ” represents the wavelength of applied light and “NAobj” represents the numerical aperture of a focusing lens focusing the light onto the data-recording medium. The resolution is also in proportion to this value. The numerical aperture NAobj is represented by the following formula:NAobj=nA×sin θwhere “nA” is a refractive index of a medium, such as air, between a lens and the data-recording medium; “θ” is an incident angle of a peripheral rays through the objective lens; and NAobj is not more than 1(one) when the medium is air, restricting the resolution. It has been attempted to shorten the wavelength of light from a light source, such as a semiconductor laser, and to increase the numerical aperture of a focusing lens in data recoding/reproducing apparatuses.
On the other hand, a near-field optical recording/reproducing method has been proposed in the art as a method for attaining a numerical aperture of more than 1. In other words, the method uses an evanescent wave, or light exhibiting exponential decay with distance from an interface. In the near-field optical recording/reproducing method, a gap between the focusing lens and the data-recording medium may need to be extremely small.
For example, Japanese Unexamined Patent Application Publication No. H5-189796 discloses a technology of irradiating a data-recording medium with near-field light to carry out a near-field recording/reproducing. In the technology, a near-field optical system includes a combination of an objective optical lens and a solid immersion lens (SIL) to carry out a near-field recording/reproducing.
FIGS. 1A, 1B, 2A and 2B illustrate examples of configuration of solid immersion lens typically used in the art, respectively. Each solid immersion lens has a super-hemispherical part and a flat or cone-shaped part facing a data-recording medium when irradiating the data-recording medium with light. FIG. 1A is a schematic cross-sectional view of a D-shaped solid immersion lens (SIL) 92 with a flat part facing an object. FIG. 1B is a schematic cross-sectional view of a focusing lens 94 including a combination of the SIL 92 and an optical lens 93. FIG. 2A is a schematic cross-sectional view of another SIL 102 with a cone-shaped part facing an object. FIG. 2B is a schematic cross-sectional view of a focusing lens 104 including a combination of the SIL 102 and an optical lens 103. FIGS. 1A, 1B, 2A and 2B show that each of the SILs 92 and 102 faces the surface of an object irradiated with light, such as a data-recording medium 111, with a small gap therebetween and that the surface of the data-recording medium 111 is being irradiated with incident light “Li”. It is noted that the optical properties of the D-shaped SIL 92 shown in FIGS. 1A and 1B are principally the same as those of the cone-shaped SIL 102 shown in FIGS. 2A and 2B, because only the shapes of the portions off the optical path of incident light are different from each other.
The spherical part of SIL is not limited to having a super-hemispherical shape. Alternatively, it may be hemispherical. If the radius of the spherical part is “r” and the refractive index of the material of the lens is “n”, then the thickness of the spherical part is “r” in the direction of the optical axis in the case of the spherical part in a hemispherical shape or “r+r/n” in the case of the spherical part in a super-hemispherical shape.
If a focusing lens is formed of a two-group lens, a combination of an optical lens and a SIL, as described in Japanese Unexamined Patent Application Publication No. H5-189796, the effective numerical aperture “NA” of the near-field optical system of the combination of lenses can be represented by the following equation (1) in the case of a hemispherical SIL:NA=nSIL×sin θi  (1)or the following equation (2) in the case of a super-hemispherical SIL:NA=nSIL2×sin θi  (2)In these equations, “θi” is an incident angle of light “Li”; and “nSIL” is a refractive index of the material of the SIL.
The above equations (1) and (2) show that an increase in refractive index of the material of the SIL, which is considered to be a medium between an objective lens and a data-recording medium, can lead to an increase in numerical aperture “NA”. In particular, an effective numerical aperture “NA” for the super-hemispherical SIL is higher than that for the hemispherical SIL when they have the same refractive index.